Electric energy storage system

ABSTRACT

An electric energy storage system having a novel structure which exhibits a ling cycle life, rapid charging-discharging characteristics and a high energy density. The electric energy storage system comprises: an anode comprised of a first material that performs interalation-deintercalation of cation as an anode active material; a cathode comprised of a second material that may form an electric doublelayer with anion as a cathode active materials; and a electrolyte including lithium salt, the electrolyte including the cation and anion. Due to a high difference between anode and cathode in capacity to store the electric energy, most electrochemical impact that occurs in the process of intercalation-deintercalation of electric energy is absorbed into cathode and active material used for anode is activated carbon having a very high resistance to electrochemical and structural impact, so that its operation life is elongated and it has rapid charging-discharging characteristics. The electric energy storage system can complement the defects of a conventional technology.

TECHNICAL FIELD

The present invention relates to an electric energy storage system, andmore particularly, to a novel electric energy storage system prepared byemploying a transitional metallic oxide including lithium as an activematerial of an anode and an activated carbon as an active material of acathode.

BACKGROUND ART

Conventionally, as the representative conventional devices for storingelectric energy, battery, capacitor, etc. may be mentioned.Specifically, a lithium rechargeable battery and an electrochemicalcapacitor are typical examples of the electric energy storage system.Since the lithium rechargeable battery has a high energy capacity, it isrecently applied widely.

The lithium rechargeable battery is recently being used as an energystorage system attached to many portable electric equipments and has ahigh energy density, so that it began to occupy market share in marketof the conventional rechargeable battery such as Ni—Cd rechargeablebattery, Ni—H battery, alkaline battery, and the like. However, thelithium rechargeable battery can not be applied to an electricautomobile, wherein the requirement rises suddenly, considering tooshort charging and discharging life time.

Recently, the lithium rechargeable battery has a charging-discharginglife that reaches about 500 times. However, in order to apply theelectric storage energy system to the electric automobile, the electricenergy storage system should have a charging-discharging life reachingmore than 100,000 times and has quick charging and discharging features.However, due to a driving principle, the electric energy storage systemhas a short cycle life and can not be promptly charged and discharged.

An electric energy storage system of the lithium rechargeable batterycan not satisfy such technical requirement. The lithium rechargeablebattery employs metallic oxide enabling electrochemicalintercalation-deintercalation of lithium as the anode material and agraphite as the cathode material.

The process of intercalation-deintercalation of lithium from the cathodeand the anode is an electrochemical reaction that is very slow and givesgreat impacts on the structure of the active material included in thecathode and the anode, so that the life of the battery is shortened.Moreover, it is known that a repeated rapid discharging-charging rapidlyshortens the cycle life thereof.

Another representative electrochemical capacitor as one of electricenergy storage system is an electric double layer capacitor (EDLC). EDLCemploys an activated carbon having a large surface area as an activematerial for the cathode and the anode, and an electrolyte including anammonium salt such as tetraammonium tetrafluoroborate, andtetraethylammonium hexafluorophosphate. These ammonium salts produceelectric double layers onto the interface of the activated carbon havinga large surface area. That is, the electric charge layers havingpolarity being different from each other are formed on the interfacebetween the electrode and the electrolyte through an electrical staticeffect. The resultant electric charge distribution is called as anelectric double layer. As a result, the surface area of the activatedcarbon has the same capacitance as a condenser.

Therefore, since the process producing the electric double layer is arapid electrochemical reaction and does not give a structural impact onthe active materials, the electric double layers show a long cycle lifeand rapid charging-discharging characters. However, the surface area ofthe activated carbon used for the active material can not be expandedinfinitely and the capacity for storing an electric energy obtained fromthe electric double layer is very low as compared with anelectrochemical oxidation-reduction reaction, so that it might beimpossible to obtain a high energy density.

As compared with the above-mentioned rechargeable battery, an EDLCexhibits character being contrary to the rechargeable battery. Namely,the EDLC shows a rapid discharging and charging characteristic, a cyclelife that is longer than a rechargeable battery, and is useful for awide temperature range, as expected from the driving principle. However,the EDLC has a fatal weak point that the energy density is very low, ascompared with a rechargeable battery.

Moreover, there is another electrochemical capacitor using a metallicoxide that shows similar characteristics to the EDLC. U.S. Pat. No.5,600,535 (issued to Jow et al.) discloses an electrochemical capacitorthat employs amorphous metallic oxide as an active material. Also, theabove patent describes that if oxidized ruthenium is used, a highcapacity of 430F/g can be obtained. However, this value means that thecapacity is higher than a conventional EDLC but does not mean thatcapacity is higher than a lithium rechargeable battery. Also, amanufacturing cost of ruthenium dioxide is very high, so that rutheniumdioxide can not be actually employed for an active material of anelectrode. Both the cathode and anode of electrochemical capacitor of ametallic oxide are composed of amorphous metallic oxide.

In the mean time, U.S. Pat. No. 6,252,762 (issued to Amatucci) disclosesa hybrid battery/super capacitor system wherein charging-discharging maybe performed. In the above system, the electrode that may performinteraction-deintercalation of ion is employed as the cathode and theone for capacity is as the anode. The above-mentioned patent discloseshigh energy density characteristics in a battery and rapidcharging-discharging characteristics and a long life-time in acapacitor. However, even in the system having such a novel structure,much improved characteristics in the energy density,charging-discharging characteristics and loner life time are required.

The present inventors disclosed a system using both lithium salt and anammonium salt as a solute of organic electrolyte entitled“Electrochemical Pseudocapacitor of Metallic Oxide Using An OrganicElectrolyte” in Korean Patent Application No. 2000-71136, which wasfiled on 2000. Nov. 28. This application is a priority application ofU.S. patent application Ser. No. 09/824,699. The above applications arepending in both countries. The above-mentioned applications disclose atechnique for introducing two kinds of salts that are applicable todifferent systems into one system. Namely, the above applicationsdisclose a system using a lithium salt applicable to a lithiumrechargeable battery and an ammonium salt applicable to a capacitor suchas EDLC simultaneously. The system exhibits satisfactory capacitycharacteristics.

According to the above-mentioned disclosure, when only one kind of saltis used, satisfactory results can not be obtained. When lithium ion isused only, an electric conductivity becomes low, so that thepseudocapacitor can not function as a capacitor. When an ammonium saltas a support electrolyte is added thereto, an electric conductivitybecomes high so that desirable results can be obtained.

DISCLOSURE OF INVENTION

In order to overcome the above problems in the conventional lithiumrechargeable battery such as short cycle life and slowcharging-discharging characteristics, and a low energy density that isone defect of an electrochemical capacitor, it is an object of thepresent invention to provide an electric energy storage system having anovel structure which exhibits a long cycle life, rapidcharging-discharging characteristics and a high energy density.

To accomplish the above object, there is provided in the presentinvention an electric energy storage system comprising:

-   -   an anode comprised of a first material that performs        interalation-deintercalation of cation as an anode active        material;    -   a cathode comprised of a second material that may form an        electric doublelayer with anion as a cathode active materials;        and    -   an electrolyte including lithium salt and ammonium salt, the        electrolyte including the cation and anion.

Preferably, the anode active material is an oxide including lithium anda transitional metal and the cathode active material includes anactivated carbon.

The above object of the present invention may be accomplished by anelectric energy storage system comprising:

-   -   an anode including a first material that performs        interaction-deintercalation of cation as an anode active        material;    -   a cathode including a second material that may form an electric        double layer with anion as a cathode active material; and    -   an electrolyte including a lithium salt, the electrolyte        including the cation and the anion.

Particularly, as the above-mentioned transitional metal, at least oneselected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Mo, and Nimay be preferably used. As an oxide including the lithium and thetransitional metal, LiCoO2, LiMnO2, LiMn2O4, LiNiO2, LiMoO2, LiV2O5,LiCoxNi1-xO2(0<x<1), and the like may be mentioned.

The electrolyte may include a lithium salt such as LIBF4, LiAsF6,LiCIO4, LiPF4, etc. in a dissolved state, and simultaneously include anammonium salt such as tetraethylammonium tetrafluoroborate((CH3,CH2,)4,NBF6), tetraethylammonium hexafluorophosphate((CH3,CH2,)4,NPF6), tetraethylammonium perclorate((CH3CH2,)4,NCIO4,) indissolved state.

The present invention can overcome all the defects in the conventionalelectric energy storage systems such as a lithium rechargeable batteryand EDLC by employing a transitional metal including lithium as an anodeactive material, activated carbon as a cathode active material, and anelectrolyte including both lithium and ammonium salt or lithium only.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic structure of a winding type cell as oneexample of an electric energy storage system according to the presentinvention.

FIG. 2 illustrates a schematic structure of a packing type cell as oneexample of an electric energy storage system according to the presentinvention.

FIG. 3 is a graph illustrating changes in the electric potentialsbetween a cathode and an anode when an electric potential is applied tothe system, wherein as an anode, a transitional metallic oxide includinglithium, LiCoO2 is used, as a cathode, BP of activated carbon is used,and as an electrolyte, an organic electrolyte prepared by dissolvingLiPF6 of 1 M and (CH3CH2)NBF4 of 1 M in acetonitrile is used, as oneexample of an electric energy storage system according to the presentinvention.

FIG. 4 is a graph illustrating a result of the same system as in FIG. 3measured by a volt scanning method.

FIG. 5 is a graph illustrating a result obtained when charging the samesystem as in FIG. 3 and then discharging the same system with 100 mA,500 mA, 1 A and 3 A.

FIG. 6 is a graph illustrating a change in capacity during the samesystem as in FIG. 3 is discharged and charged 10,000 times at 1 V-2.3 V.

FIG. 7 is a graph illustrating changes in capacity according tofrequencies of charging-discharging in a conventional lithiumrechargeable battery and the system as shown in FIG. 3.

FIG. 8 is a graph illustrating changes in capacity according tofrequency of charging-discharging in system (b) of comparative example 4and the system as show in FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The present will be described in detail with reference to the attacheddrawings below.

FIG. 1 illustrates a schematic structure of a winding type cell as oneexample of an electric energy storage system according to the presentinvention. In the figure, a denotes an anode, b denotes a separatinginsulation membrane and c denotes a cathode.

FIG. 2 illustrates a schematic structure of a packing type cell as oneexample of an electric energy storage system according to the presentinvention. In the figure, a denotes an anode, b denotes a separatinginsulation membrane and c denotes a cathode.

The driving method of the electric energy storage system according tothe present invention is as follows:

In the anode, an electric charge and discharge occurs byoxidation-reduction reaction resulted from electrochemicalintercalation-deintercalation of lithium ions included in an electrolyteand in the transitional metallic oxide including lithium. In thecathode, an electric charge is stored and discharged simultaneously withthe anode by a fact that ammonium ion forms and removes an electricdouble layer on the surface of an activated carbon that is used as acathode active material. Therefore, the electric energy storage systemof the present invention exhibits a high energy density, a long life andrapid charging-discharging characteristics.

However, in case of the anode, the capability of storage-discharge ofelectric energy per unit weight is much higher than the cathode due tothe difference in the driving principles.

For example, when a driving voltage is applied to an anode and a cathodehaving activated materials in the same quantity, the voltage of acathode, whose capability to store energy is considerably lower than theanode, is changed rapidly, but the voltage of an anode is scarcelychanged. Namely, the capability to store the electric energy of an anodeis superior to the maximum value of the cathode. Also, the capability isused in a degree far less than the capability of an anode, so that astructural impact is lessened. Therefore, both long charging-discharginglife and rapid charging-discharging characters are shown.

According to the present invention, a capability to store electricenergy is expanded by using a high energy density of a transitionalmetallic oxide including lithium as an anode active material. Also, along life cycle and rapid charging-discharging are shown by a fact thatan activated carbon used for a cathode active material absorbs impactsapplied to the active materials.

In the present invention, an electrolyte having a lithium salt only orboth a lithium salt and ammonium ion is necessary. There is a bigdifference in efficiency between an electrolyte having both lithium saltand ammonium salt and an electrolyte having lithium or ammonium saltonly. When an ammonium salt is used only, an initial capability to storeelectric energy is so low to correspond to approximately half of a casewhen mixed salts are used. Also, charging-discharging life isdrastically dropped. This is because that the size of ammonium salt inthe electrolyte is very large, so that the ammonium salt can notparticipate in electrochemical charging-discharging reaction and duringthe system is driven, the structure of an anode material is broken by aninsertion reaction of the ammonium salt into an anode material by thevoltage.

When only a lithium salt is used, the initial capability to storeelectric energy is somewhat dropped so as to reach approximately 90% ascompared with a case of using a mixed electrolyte. However, this valueis higher than using ammonium salt only. Therefore, using lithium saltonly is also incorporated in the present invention.

A part of a transitional metal of an active material used for an anodeof the system in the present invention can be replaced with Al, B, Ca,Sr, Si, etc., and the replaced quantity is preferably no more than 30%by mole. In case of a conventional lithium rechargeable battery, if apart is replaced with the above mentioned materials, a capability isenhanced by approximately 20% or less in view of cycle life. In order toconfirm whether or not such enhancement of capability is shown in thissystem, the present inventors replace a part of the transitionalmaterial with the above materials when manufacturing active materials.The replacement may slightly enhance the capability, but suchreplacement may be also included in the present invention.

Likewise, oxygen in the anode active material can be partially replacedwith S, I, F, Cl, Br, etc. It should be understood that all changes inthe materials due to such replacement in a small quantity is included inthe present invention.

The specific surface area of the anode active material is preferably noless than 200 m²/g. Generally, the capability to store energy byactivated carbon is in proportional to the surface area of activatedcarbon, so that the wider the surface is, the more energy can be stored.Therefore, in case where the surface area of activated carbon is small,the capability to store energy is not high. The wider the surface areais, the more capability to store energy is enhanced. However,considering an economical efficiency, activated carbon having a specificsurface area of about 500-2000 m²/g is preferably used.

When selecting electrode materials for an anode or cathode, an anodeemploys an electrode using a transitional metallic oxide includinglithium as an anode active material and a cathode employs an electrodeusing activated carbon as a cathode active material. On the contrary, asystem, wherein a cathode employing an electrode using a transitionalmetallic oxide including lithium as a cathode active material and ananode employing an electrode using activated carbon as an anode activematerial, is excluded. The reason is based on the electrochemicalreaction mechanism generated from each electrode pole. The detailedexplanation is as follows.

In the anode, charging means that voltage increases toward (+)direction. On the contrary, in the cathode, charging means that voltageincreases toward (−) direction. At this time, cations having samepolarity are forced towards a direction of far from anode by a repulsiveforce. These cations include not only cations of a salt dissolved in theelectrolyte but also cations included in the anode active material.However, not all cations included in the active material can movefreely. In only a specific case, cations can move. For example, in caseof LiCoO2 comprised of two kinds of cations, Li and Co ions, Co ionsforms a frame of the material, so that Co ions exist in a fixed state,but Li ions can freely come into and out of the frame formed by Co and Oions. Therefore, Li ions can move according to the polarity of voltageapplied to the electrode.

In the anode, charging is a phenomenon that cation moves far from theanode and discharging is a phenomenon that cation moves toward theanode. If material such as LiCoO2 that may function to store an electricenergy by intercalation-deintercalation of lithium ion is employed as ananode active material, Li ion is released during charging and Li ion isincorporated into the active material during discharging.

Since the reaction mechanism for storing an electric energy is asabove-mentioned, the important items to be considered is a compositionof material to be used as an active material. For example, LiCoO2 can beused as a cathode active material, but is not suitable for an anodeactive material. This is because a quantity of Li ion included in LiCoO2is one mole and no more Li ions can be included therein. Namely, Licontent in LiCoO2 is under a saturated state. Therefore, if LiCoO2 isemployed as an anode and voltage of (+) pole is applied thereto duringinitial charging, Li ion can be released from the inside of the activematerial. On the contrary, if LiCoO2 is employed as a cathode andvoltage of (−) pole is applied thereto during initial charging, Li ionscan not be more incorporated into the active material.

If LiCoO2 is used as cathode and voltage of (−) pole is applied thereto,Li ions can not be moved into LiCoO2, so that the quantity of electricenergy to be stored is pretty reduced and especially, an impact on thestructure of the active material becomes big, so that the stabilityaccording to repeated charging-discharging operation decreases rapidly.

Activated carbon can be used for both electrode as an active materialsof electric energy storage system regardless of the poles of electrodes.This is because it is different from an oxide including Li inelectrochemical reaction system. In the oxide including Li, only Li ionscan participate in process for storing electric energy. Namely, ions inthe electrolyte wherein LiPF6 is dissolved are Li+ ion and PF6− ion.However, only Li ions participate in the reaction, thus the electricpotential applied to the electrode acts as a very important factor.

However, the electrochemical reaction mechanism generated from activatedcarbon is a phenomenon of an electric double layer and both cation andanion ions can participate in this phenomenon, so that activated carboncan be applied to cathode or anode. Electric energy can be storedthrough forming an electric double layer by using anion or cation.During charging reaction of anode, (+) pole is applied, so that anion isused and during charging reaction of cathode, (−) pole is applied, sothat cation is used. In this manner, the electric double layer is formedfor storing an electric energy.

The electric energy storage system is manufactured as follows.

First, a conducting agent and a binder are added into a transitionalmetallic oxide including lithium such as LiCoO2, LiMnO2, LiMn2O4,LiNiO2, LiMoO2, LiV2O5, LiCoXNi1-XO2(0<x<1) and the like and then mixed.An anode is manufactured by coating the resultant on a surface of a thinmetallic panel such as Al, Ni, Cu and the like. Preferably, theconducting agent and the binder in a predetermined amount are added toactivated carbon, such as BP, MSC, MSP, YP (trade names; BP and YP aremanufactured by Kuraray Co., Ltd. of Japan and MSC and MSP aremanufactured by Kansai Cobes Co., Ltd. of Japan) having a specificsurface area more than 200 m²/g and then mixed. A cathode ismanufactured by coating the resultant on the surface of a thin metallicplate such as Al, Ni, Cu and the like.

An electrolyte wherein a lithium salt such as LiBF4, LiAsF6, LiClO4,LiPF6 and the like and an ammonium salt such as tetraethylammoniumtetrafluoroborate, tetraethylammonium hexafluorophosphate,tetraethylammonium perchlorate and the like are dissolved in a ratio of5:5 by mole is prepared. Particularly, a mixed ratio of the lithiumsalts with respect to the ammonium salt is about 4:6˜6:4. The electricenergy storage system of the present invention is manufactured byinserting a separating insulation membrane including the aboveelectrolyte between the two electrodes. When the electrolyte isprepared, adding an ammonium salt in addition to the lithium saltresults in an enhanced capability to store the electric energy. This isbecause that the ammonium ion that is bigger than the lithium ion mayenhance the effect on the formation of the electric double layer.

The preferred examples of the present invention will be described inmore detail.

EXAMPLE 1

As an anode active material, LiCoO₂ including lithium was used and as ancathode active material, BP (trade name manufactured by Kuraray Co.Ltd., Japan), a kind of activated carbon was used. After the activematerials of each electrodes ware mixed with conductive carbon in aratio of 8 to 2 by weight, water that includes a binder PVdF of 10 wt. %in a dissolved state was added thereto and then they were mixed, so asto prepare slurry. An aluminum foil having a thickness of 20 mm wascoated with the resultant slurry, and then the coated aluminum foil wasdried in a dryer at a temperature of 120° C., to complete an electrode.

Thus prepared electrodes were assembled together, by interposing aseparating insulation membrane therebetween as shown in FIG. 1. Theelectrolyte was comprised of acetonitrile as a solvent and LiPF₆ of 1.0M and tetraethylammonium tetrafluoroborate of 1.0 M as a solute. At thistime, the surface area of each of the electrodes was 150 cm² and theassembled body of the electrodes and the separating insulation membranewas inserted into an aluminum cylinder having 10.2 cm³ in volume andthen was sealed.

EXPERIMENTAL EXAMPLE 1

When voltage of 2.5V was applied to both anode and cathode of theelectric energy storage system prepared in Example 1, the respectivemeasured values of the voltage applied to anode and cathode are shown inFIG. 3.

During the electric potential is applied from 1V to 2.5V, the electricpotential applied to the anode was actually changed from 4.1V to 4.8 Vvs. Li/Li+ and the electric potential to cathode was changed from 3.08Vto 1.69 V vs. Li/Li+.

As a result, changes in the electric potentials in the present systemwere almost observed on the cathode. Accordingly, it can be noted thatan electrochemical impact when storing an electric energy has occurredon the cathode not the anode. This is the reason why a structurallyfragile anode can be protected and the present electric energy storagesystem has a long life time and rapid charging and dischargingcharacters.

EXPERIMENTAL EXAMPLE 2

The measured CV value of the electric energy storage system prepared inExample 1 by a voltage scan method is shown in FIG. 4. As illustrated inFIG. 4, the measured value of CV is similar to that of anelectrochemical capacitor.

EXPERIMENTAL EXAMPLE 3

After the electric energy storage system prepared in Example 1 wasdischarged at 2.5V in the above electrolyte, the potential voltagesshown during discharging at 100 mA, 500 mA, 1 A and 3 A were illustratedin FIG. 5. When calculated in a capacitance unit, the dischargingcapacitance reaches 139F that is high. Also, even at a high current of 3A, the electric energy storage system can operate sufficiently.

EXPERIMENTAL EXAMPLE 4

When an electric energy storage system prepared in Example 1 was chargedand discharged continuously at 2.3-1.0 V with an electric current of 3 Ain the above electrolyte, the changes in the capacities of the electricenergy storage system are illustrated in FIG. 6. Although thecharging-discharging time reaches 10,000, an excellent cycle life isshown, such that more than 80% of an initial capacity can be maintained.

EXAMPLES 2-4

The same procedures were repeated as in Example 1, except thattetraethylammonium tetrafluoroborate of 1.0 M as a solute was unchangedand different kinds of lithium salts such as LiBF₄ (Example 2), LiCIO₄(Example 3) and LiAsF₆ (Example 4) were used, thereby preparing anelectric energy storage systems. Electric energy storage capacity whenthe systems were charged at 2.5V and then discharged at 0.1 A is shownin Table 1. As can be noted from Table 1, high capacities to storeelectric energy more than 130F were shown in all cases. TABLE 1 ExampleExample 2 Example 3 Example 4 Lithium salt LiBF₄ LiClO₄ LiAsF₆ Electricenergy storage capacity 138 128 132 (F)

EXAMPLES 5-7

The same procedures were repeated as in Example 1, except that LiPF₆ of1.0 M as a solute was unchanged and different kinds of ammonium saltssuch as tetraethylammonium tetrafluoroborate (Example 5),tetraethylammonium hexafluorophosphate (Example 6) andtetraethylammonium perchlorate (Example 7) were used, so as to preparean electric energy storage systems. Electric energy storage capacitieswhen the systems were charged at 2.5V and then discharged at 0.1 A areshown in Table 2. As can be noted from Table 2, high capacities to storean electric energy more than 120F were shown in all cases. TABLE 2Example Example 5 Example 6 Example 7 Ammonium (CH₃CH₂)₄NBF₄(CH₃CH₂)₄NPF₆ (CH₃CH₂)₄NclO₄ salt Electric energy 139 139 135 storagecapacity (F)

EXAMPLES 8-12

The same procedures were repeated as in Example 1, except that LiMn2O4(Example 8), LiMnO2 (Example 9), LiNiO2 (Example 10), LiCo0.8Ni0.2 O2(Example 11), and LiA10.01Mn1.99O3.98S0.02 (Example 12) were used as ananode active materials, so as to prepare electric energy storagesystems. Electric energy storage capacities when the systems werecharged at 2.5V and then discharged at 0.1 A are shown in Table 3. Asnoted from Table 3, high capacities to store an electric energy wereshown in all cases. TABLE 3 Example Example Example 8 Example 9 10Example 11 Example 12 Anode LiMn₂O₄ LiMnO₂ LiNiO₂ LiCo_(0.8)Ni_(0.2)O₂LiAl_(0.01)Mn_(1.99)O_(3.98)S_(0.02) Material Electric 135 132 127 136133 energy storage capacity (F)

EXAMPLES 13-15

The same procedures were repeated as in Example 1, except that MSC(Kansai Cobes Co. Ltd., Japan, Example 13), MSP (Kansai Cobes Co. Ltd.,Japan, Example 14), and YP(Kuraray Co., Ltd., Japan, Example 15),instead of BP as activated carbon were used as a cathode activematerial, so as to prepare electric energy storage systems. Electricenergy storage capacities when the systems were charged at 2.5V and thendischarged at 0.1 A are shown in Table 4. As noted from Table 4, highcapacities to store an electric energy were shown in all cases. TABLE 4Example Example 13 Example 14 Example 15 cathode material MSC MSP YP

EXAMPLE 16

The same procedure was repeated as in Example 1, except that anelectrolyte using LiPF6 of 1 M as a solute, instead of both LiPF6 and(CH3CH2)NBF4 of 1 M, was employed, so as to prepare an electric energystorage system. When the system was charged at 2.5V and then dischargedat 0.1 A, an electric energy storage capacity is shown in Table 5. In acase where a lithium salt is used only, sufficiently a high capacity tostore an electric energy was shown, although the value is somewhat lowerthan the case when using two kinds of salts simultaneously.

COMPARATIVE EXAMPLE 1

The procedure was repeated as in Example 1, except that an electrolyteusing tetraethylammonium tetrafluoroborate of 1 M only as solute,instead of both tetraethylammonium tetrafluoroborate and LiPF6 of 1M,was employed, so as to prepare an electric energy storage system. Whenthe system was charged at 2.5V and then discharged at 0.1 A, an electricenergy storage capacity is shown in Table 5. In a case where only anammonium salt is used, a somewhat low capacity to store an electricenergy was shown. TABLE 5 System Example 16 Comparative Example 1 usedsalt LiPF₆ (CH₃CH₂)₄NBF₄ Electric Energy storage 125 86 Capacity (F)

EXPERIMENTAL EXAMPLES 5-7

After charging-discharging the electric energy storage systems preparedaccording to Example 16, Comparative Example 1 and Example 1, thechanges in capacities were observed. Each of the electric energy storagesystems was continuously charged-discharged at 2.3-1.0V with an electriccurrent of 3 A. After charging-discharging operations were performed20,000 times, the observed changes in capacity are shown in Table 6. Incase of an electric energy storage systems according to Example 16 andComparative example 1 in which only one kind of salt was used, althoughthere is a somewhat difference, after charging-discharging 20,000 times,65-83% of the initial electric energy storage capacity reduced, so thatthey are expired. On the contrary, in case of an electric energy storagesystem according to Example 1, wherein an electrolyte is prepared bymixing two kinds of solutes, only 14% of the initial electric energystorage capacity was reduced under the same condition. Therefore, it canbe noted that a case where lithium salt and ammonium salt are mixedtogether is superior to another case in performance.

As compared with cases using only one kind of a salt, in a case of usingammonium salt only or lithium salt only, respectively, 83% or 65% of theinitial electric energy storage capacity was reduced. Therefore, if onekind of salt is used, a case of using lithium salt only is more superiorto another case wherein one salt is used. TABLE 6 Experimental ExampleExperimental Experimental Experimental Example 5 Example 6 Example 7Solute LiPF₆ (CH₃CH₂)₄NBF₄ LiPF₆/(CH₃CH₂)NBF₄ composition Reduction −65−83 −14 ratio of electric energy storage capacity (%)

COMPARATIVE EXAMPLE 2

In order to compare the energy storage system in accordance with thepresent invention with a conventional lithium rechargeable battery incycle life, a lithium rechargeable battery was prepared by employingLiCoO2 and graphite as active materials of anode and cathode,respectively and LiPF6 as a solute of an electrolyte.

FIG. 7 is a graph illustrating changes in capacities according to cyclefrequency with respect to thus obtained lithium rechargeable battery.Since a critical life of lithium rechargeable battery is about 500cycle, the changes in capacities corresponding to low number of cycleswere illustrated. In the figure, graph “a” illustrates changes incapacities corresponding to a low cycle frequency after the experimentwas accomplished according to Experimental Example 4 with the presentelectric energy storage system manufactured in Example 1. Graph “b”illustrates changes in capacities corresponding to cycle frequency inthe lithium rechargeable battery manufactured in Comparative Example 2.

However, since the driving condition of the present energy storagesystem is completely different from lithium rechargeable battery, theexperimental conditions are somewhat different from each other. Forexample, while the voltage-driving boundary of the present invention was2.5 V, that of lithium rechargeable battery was 4.2V. Therefore,although direct comparison under the exactly same condition isunreasonable, conditions suitable for an original driving purpose hasbeen considered. Thus, it can be regarded as being comparable.

COMPARATIVE EXAMPLE 3

In order to compare the energy storage system of the present inventionwith the capacity of a conventional EDLC, EDLC was prepared by using MSCas an active material and a solution wherein tetraammoniumtetrafluoroborate of 1.0 M had been dissolved in acetonitrile. The sameexperiment as in Experimental Example 3 was repeated, except that theobservation was obtained when it was discharged with 100 mA. When it isconverted into capacity, it showed capacity to store an electric energyof approximately 47F. This value is very low one, as compared with theenergy storage system manufactured in Example 1 and such comparison isshown in table 7. TABLE 7 Comparative Comparative System example 3example 1 Electric energy storage capacity 47 139 (F)

COMPARATIVE EXAMPLE 4

As a cathode active material of cathode, LiCoO2 including lithium and asan anode active material, BP (Kuraray Co. Ltd., Japan), a kind ofactivated carbon was used. After active materials of each electrodeswere mixed with conductive carbon in a ratio of 8 to 2 by weight, waterincluding a binder PVDF of 10 wt. % in a dissolved state was addedthereto and then the resultant was mixed, to obtain slurry. An aluminumfoil having a thickness of 20 mm was coated with the resultant slurry,and then it is dried in dryer at temperature of 120° C., to completeelectrodes.

Thus prepared electrodes were assembled together, by interposing aninsulation membrane therebetween as shown in FIG. 1. The electrolyte wascomprised of acetonitrile as a solvent and LiPF6 of 1.0 M andtetraethylammonium tetrafluoroborate of 1.0 M as a solute. At this time,the surface area of each electrode was 150 cm², and thus assembled bodyof electrodes and a separating insulation membrane was inserted into analuminum cylinder having 10.2 cm³ in volume and then was sealed.

Electric energy storage systems prepared in Example 1 and ComparativeExample 4 were charged at 2.3V and then discharged at a current of 0.1 Aand then the resultant accumulated electric energy amount was observed.This result is shown in Table 8. TABLE 8 System Example 1 Comparativeexample 4 Constitution of anode LiCoO₂ (+)/ Activated carbon (−)/ andcathode activated carbon (−) LiCoO₂ (+) Electric energy capacity 139 47(F)

As can be noted from Table 8, when an electrode using cobalt oxideincluding lithium as an active material was used as anode and anelectrode using activated carbon as an active material was used ascathode, a very high electric energy storage capacity reaching 139F wasshown. However, when wired conversely, namely, when an electrode usingcobalt oxide including lithium as active material was used as cathodeand an electrode using activated carbon as an active material was usedas anode, a very low capacity of electric energy reaching 18F was shown.This value was very low so as to correspond to 13% thereof and is alsolow as compared to EDLC that uses activated carbon for both electrodes.

FIG. 8 is a graph illustrating comparatively changes in capacities dueto charging-discharging frequency in the same system as in FIG. 3prepared in Example 1 and the system according to Comparative Example 4.Namely, this shows changes in capacities when they were continuouslycharged and discharged at 2.3-1.0V with an electric current of 3 A.

As can be seen from graph “a” of FIG. 8, when anode employs an electrodeusing cobalt oxide including lithium as an anode active material andcathode employs electrode using activated carbon BP as a cathode activematerial, repeatedly charging-discharging 100 times does not affectcapacities to store electric energy at all. As noted from graph “b” ofFIG. 8, however, when wired reversely, namely, a cathode employs anelectrode using cobalt oxide including lithium as a cathode activematerial and an anode employs an electrode using activated carbon BP asan anode active material, repeatedly charging-discharging 100 timesresults in loss of an electric energy of 40%.

Generally, considering that the frequency of repeatedcharging-discharging of the present electric energy storage system is atleast 10,000 times, when wired reversely, namely, when the cathodeemploys an electrode using cobalt oxide including lithium as a cathodeactive material and the anode employs an electrode using activatedcarbon BP as an anode active material, it can be confirmed that a normaloperation is hard to be expected.

When the anode employs a transitional metallic oxide including lithiumand the cathode employs activated carbon and an electrolyte includesboth lithium salt and ammonium salt, as in the present system, defectsinherent in a lithium rechargeable battery and an EDLC, so-calledconventional representative electric energy storage systems can beremoved and following characteristics can be obtained.

Firstly, electric energy capacities which can be store an electricenergy per unit volume or unit mass can be surprisingly enhanced byusing high capacity to store an electric energy of a transitionalmetallic oxide including lithium used for anode.

Then, charging-discharging life characteristics being far superior tothe conventional lithium rechargeable battery can be guaranteed. Due toa high difference between anode and cathode in capacity to store theelectric energy, most electrochemical impact that occurs in the processof intercalation-deintercalation of electric energy is absorbed intocathode and active material used for anode is activated carbon having avery high resistance to electrochemical and structural impact, so thatits operation life is elongated and it has rapid charging-dischargingcharacteristic.

Finally, the present electric energy storage system, which cancomplement the defects of a conventional technology, is characterized inthat it has much longer life time than the conventional lithiumrechargeable battery; it has rapid charging-discharging features; and ithas much higher capacity to store energy than the conventionalelectrochemical capacitor.

While the present invention is described in detail referring to theattached embodiments, various modifications, alternate constructions andequivalents may be employed without departing from the true spirit andscope of the present invention.

1. An electric energy storage system comprising: an anode including afirst material that performs interalation-deintercalation of cation asan anode active material; a cathode including a second material that mayform an electric double layer with anion as a cathode active material;and an electrolyte including the cation and the anion, said electrolyteincluding both lithium salt and ammonium salt.
 2. An electric energystorage system as claimed in claim 1, wherein said anode active materialincludes an oxide comprised of lithium and a transitional metal.
 3. Anelectric energy storage system as claimed 2, wherein said transitionalmetal is at least one selected from the group consisting of Ti, V, Cr,Mn, Fe, Co, Mo and Ni.
 4. An electric energy storage system as claimed2, wherein said oxide is at least one selected from the group consistingof LiCoO2, LiMnO2, LiMn2O4, LiNiO2, LiMoO2, LiV2O5 andLiCoXNi1-XO2(0<x<1).
 5. An electric energy storage system as claimed 1,wherein said cathode active material includes activated carbon.
 6. Anelectric energy storage system as claimed 5, wherein a specific surfacearea of said activated carbon is no less than 200 m²/g.
 7. An electricenergy storage system as claimed 1, wherein said lithium salt is atleast one selected from the group consisting of LiBF4, LiAsF6, LiClO4and LiPF6.
 8. An electric energy storage system as claimed 1, whereinsaid ammonium salt is at least one selected from the group consisting oftetraethylammonium tetrafluoroborate ((CH3CH2)4NBF6), tetraethylammoniumhexafluorophosphate ((CH3CH2)4NPF6), and tetraethylammonium perclorate((CH3CH2)4NCIO4).
 9. An electric energy storage system comprising: ananode including a first material that performsintercalation-deintercalation of cation as an anode active material; acathode including a second material that may form an electric doublelayer with anion as a cathode active material; and an electrolyteincluding lithium salt, the electrolyte including the cation and anion.10. An electric energy storage system as claimed in claim 9, whereinsaid anode active material includes an oxide comprised of lithium and atransitional metal.
 11. An electric energy storage system as claimed 10,wherein said transitional metal is at least one selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Mo and Ni.
 12. An electric energystorage system as claimed 10, wherein said oxide is at least oneselected from the group consisting of LiCoO2, LiMnO2, LiMn2O4, LiNiO2,LiMoO2, LiV2O5 andLiCoXNi1-XO2(0<x<1).
 13. An electric energy storagesystem as claimed 9, wherein said cathode active material includesactivated carbon.
 14. An electric energy storage system as claimed 9,wherein a specific surface area of said activated carbon is no less than200 m²/g.
 15. An electric energy storage system as claimed 9, whereinsaid lithium salt is at least one selected from the group consisting ofLiBF4, LiAsF6, LiClO4 and LiPF6.
 16. An electric energy storage systemas claimed 9, wherein said ammonium salt is at least one selected fromthe group consisting of tetraethylammonium tetrafluoroborate,tetraethylammonium hexafluorophosphate and tetraethylammoniumperchlorate.